A passive optical network (PON) is a mainstream technology in the broadband access field at present. The PON includes an optical line terminal (OLT) installed on a central control station and multiple optical network units (ONU) installed on a user site.
In the prior art, the PON can transmit uplink data and downlink data separately by using two polarizations of an optical signal which are perpendicular to each other. Specifically, the downlink data is modulated on a horizontal component of an optical signal sent by the OLT to the ONU, and a vertical component is used for uplink data modulation. After receiving the optical signal, the ONU needs to divide the optical signal into the vertical component and the horizontal component, and perform demodulation processing for the optical signal on the horizontal component to acquire the downlink data sent by the OLT. In addition, the ONU needs to modulate uplink data to be sent to the OLT onto the vertical component and send the optical signal to the OLT.
However, a direction of the optical signal deflects randomly in a process of transmitting an optical signal from the OLT to the ONU, and a polarization beam splitter (PBS) in the ONU only identifies a polarization in a vertical direction and a polarization in a horizontal direction. Therefore, the PBS cannot identify two polarizations of the optical signal after a random deflection and further cannot perform subsequent downlink data demodulation and uplink data modulation. If the optical signal deflects by 90 degrees in the transmission process, the PBS in the ONU will sends a component, in the optical signal, on which the downlink data is modulated incorrectly to a modulator for uplink data modulation, thereby causing an uplink data modulation error. In addition, the PBS also sends a component, in the optical signal, which is used for uplink data modulation incorrectly to a demodulator for downlink data demodulation, so that the downlink data cannot be demodulated.